An engine-driven working machine, such as a chain saw, a bush cutter, and a hedge trimmer, is known.
A working machine has an engine, an actuating part (for example, a chain with cutting edges in a chain saw), and a centrifugal clutch disposed between the engine and the actuating part. The centrifugal clutch is configured to connect the engine with the actuating part when a rotation speed of the engine is higher than a predetermined clutch-in rotation speed so that rotations of the engine are transmitted to the actuating part.
The engine of the working machine is designed so that when a throttle valve of a carburetor provided in the engine is in a fully-closed position, the engine rotates stably at a rotation speed which is lower than the clutch-in rotation speed. This state is referred to as an idling state.
When the engine is started, especially in an insufficient warming-up state, in order to stabilize starting and running operations of the engine, the engine is generally started with the throttle valve of the carburetor in a half-opened position to increase an amount of air supplied to the engine and runs while the throttle valve is maintained in the half-opened position. This state is referred to as a fast idling state.
Generally, when the engine is started, a brake for the actuating part is actuated in order to prevent the actuating part from being actuated unintentionally. In the fast idling state, since the rotation speed of the engine may become higher than the clutch-in rotation speed due to the throttle valve in the half-opened state, the brake is preferably used for safety. However, if the engine is started without using the brake in the fast idling state, the actuating part could be actuated at the starting. In order to enable the starting operation to be in safe even in this situation, the engine is provided with a controller which can operate in a rotation speed limitation mode in which the rotation speed of the engine is prevented from becoming higher than the clutch-in rotation speed after the starting operation.
The controller operates in the rotation speed limitation mode from the starting of the engine. Concretely, the controller detects the rotation speed of the engine, and when excess of the rotation speed of the engine over the clutch-in rotation speed is expected (when the rotation speed of the engine exceeds a predetermined rotation speed which is lower than the clutch-in rotation speed), the controller prevents rotation speed of the engine from becoming higher than the clutch-in rotation speed by performing a misfiring cycle which makes an ignition device of the engine inoperative or controlling an ignition timing of the ignition device.
On the other hand, when the operation of the controller is maintained in the rotation speed limitation mode, even if the throttle valve is moved to a fully-opened position, the rotation speed of the engine is prevented from becoming higher than the clutch-in rotation speed so that the centrifugal clutch is not operated and the rotations of the engine are not transmitted to the actuating part. Therefore, in order to operate the working machine, it is necessary to cancel the rotation speed limitation mode at an appropriate timing.
Providing a mechanical switch in the throttle valve is considered as one means so that when the mechanical switch detects an operation of the throttle valve, the rotation speed limitation mode is canceled. However, detecting the operation of the throttle valve without providing the mechanical switch is preferable due to possibility of malfunction of the mechanical switch and an increasing cost.
In a technology disclosed in the Patent Publication 1 indicated below, the rotation speed limitation mode is canceled by detecting a state of the engine when the throttle valve is moved to a fully-opened state.
In a technology disclosed in the Patent Publication 2 indicated below, after the throttle valve is moved to a fully-closed position to finish the fast idling state, the rotation speed limitation mode is canceled by detecting a matter that the engine becomes an idling state, that is, by detecting a matter that a time enough to decrease the rotation speed has passed.